Robin2:
To my mind it will be essential to operate the base station and the relay station on different channels (frequencies). - but I don't know if that concept exists with a HC12.
You can set the channels on HC12s but its really a once off type setup, they do not have the ability to rapidly shift channels or frequencies.
There is no error checking of the serial stream either, so you would need to add your own checksums.
There are low cost radio modules that are frequency agile with built in error checking (so send and ack\nack is possible) that are around the same price of a HC12 and they go very much further to boot.